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Just after 6 AM local time on Tuesday in Japan, a sound like an explosion was heard near the suppression pool of reactor No. 2 at the stricken Fukushima Daiichi nuclear power plant. This followed an explosion March 11 that ripped the roof off reactor No. 1 and another at reactor No. 3 on March 14 that injured 11 workers. The culprit in all three cases is likely a build-up of explosive hydrogen gas—as occurred at Three Mile Island in the U.S. in 1979 as a result of the meltdown there—caused by nuclear fuel rods experiencing extremely high temperatures stripping the hydrogen out of the plant's steam.

"The hydrogen accumulates outside of containment but inside the reactor building. You get enough and some spark source and you get an explosion," explains nuclear engineer Michael Golay of the Massachusetts Institute of Technology. "The [radioactive] cesium and iodine showing up in releases shows the fuel has been damaged."

The 3.7-meter-long nuclear fuel used at Fukushima is composed of uranium oxide pellets encased in a zirconium cladding. Though control rods have stopped the uranium fission process that drives normal operation of a nuclear reactor, the byproducts of that continue to split and generate heat. If the fuel rods are no longer being cooled—as has happened at all three reactors at the Fukushima Daiichi power plant operating at the time of the earthquake—then the zirconium cladding will swell and crack, releasing the uranium fuel pellets and fission byproducts, such as radioactive cesium and iodine, among others.

The high temperatures that the fuel rods create boil water and continually turn it into steam. If no fresh water is introduced to cool the rods then they continue to heat up. Once the rods reach more than 1200 degrees Celsius, the zirconium will interact with the steam and split the hydrogen from the water. That hydrogen can then be released from the reactor core and containment vessel and, if it accumulates in sufficient quantities—concentrations of 4 percent or more in the air—it can explode, as has apparently occurred at reactors No. 1 and 3, and possibly No. 2 as well. The explosions at reactors No. 1 and 3 destroyed the surrounding buildings but have apparently not damaged the massive steel containment vessel—as much as 20 centimeters thick—that surrounds each reactor's nuclear core.

But the explosion at reactor No. 2 is potentially more serious as it seems to have affected pressure in the suppression pool—a massive pool of water stored in a torus-shaped chamber beneath the reactor itself that both cools and captures any escaping radioactive materials. If the suppression pool is damaged or cracked and can no longer help cool the reactor—and 2.7 meters of the fuel rods in that reactor remain exposed, according to Tokyo Electric Power Company—radioactive material might escape.

And if the temperature continues to rise—and the reaction of zirconium and oxygen produces yet more heat—the cladding itself can spontaneously combust. "If the temperature gets high enough then you get a self-propagating reaction and a fire that would burn like a sparkler," says physicist Arjun Makhijani, president of the Institute for Energy and Environmental Research. "If it catches fire, all bets are off."

The TEPCO operators of the stricken nuclear power plant—a minimal staff of 50 as all non-essential personnel have been evacuated—now face a balance between venting the steam building up in the reactor (since the main danger for a widespread release of radioactive material is steam bursting the thick steel container holding the reactor) and keeping any radioactive materials inside the power plant itself. "There is no way to carry the heat away to cool down the core unless the steam that is generated is vented," Makhijani notes. "That's going to be radioactive steam."

Filters should capture some of the radioactive materials, according to Richard Meserve, president of the Carnegie Institution for Science and former chairman of the U.S. Nuclear Regulatory Commission. "The release of that steam is the way they're getting rid of energy" in order to keep cooling the damaged nuclear fuel rods, he explains.

TEPCO is currently injecting seawater and boric acid—boron absorbs the neutrons that radioactive materials give off to prevent any self-sustaining fission—into two of the three reactors, according to the company's statements. If such cooling is not continued, a full meltdown could occur in which the uranium fuel pellets escape the cladding and form a molten pool in the reactor core—potentially melting through the thick steel containment vessel. "I don't see that they have much choice but to do what they're doing," Makhijani says. If the molten pool melts through the steel containment vessel, large amounts of radiation would be released into the environment.

The Japan Atomic Industrial Forum, an industry body, estimates that core cooling systems are not functioning at all three Fukushima Daiichi operating reactors and two of the four reactors at the nearby Fukushima Daini nuclear power plant are relying on backup cooling systems.

What remains unclear is how much of the nuclear fuel at any of the three Fukushima Daiichi reactors has melted down, though TEPCO has announced that the fuel is likely damaged in all three reactors that were operating there at the time of the earthquake. A TEPCO spokesman said in a press conference on March 15 that the company "cannot deny the possibility that fuel rods may be melting." The truth of the matter may not become clear for years. After all, it took years for the U.S. Nuclear Regulatory Commission to determine that a meltdown had occurred in Three Mile Island—an effort that required sending in robots with cameras.

Already, radiation at undisclosed levels and in undisclosed forms has been detected on the U.S.S. Ronald Reagan, according to the U.S. Navy, an aircraft carrier that had been 100 miles northeast of the stricken nuclear power plant off the coast of Japan on March 14. Levels detected in a short duration pulse at the plant itself have reached as high as 8,217 microSieverts per hour, or eight times the dose endured in a typical CT scan and four times the normal dose of background radiation in a year. "They're getting out of the way of the plume," Golay says. "There is no reason to stay in the way if they do not have to."

And cooling will need to continue at the Fukushima nuclear power plants for a long time to come. After all, the spent fuel pools that may have been exposed by the power plant explosions contain more than 200 metric tons of used uranium fuel rods that have been cooling for weeks, months or even years—and smoke or steam continues to billow from the exposed spent fuel pool of reactor No. 3. "Their goal is to keep everything stable and keep the radiation bottled up in the reactor vessel," Golay says.

TEPCO continues to pump seawater and boric acid into all three reactors, particularly aiming to cover the partially exposed fuel rods in reactor No. 2, but it is proving difficult to maintain. "They will have to keep on retaining cooling for months," Meserve adds. But "if they can get through the week with this system without a major environmental release, the danger will be less."